Pentacoordinated boron species

The anionic pentacoordinated and even hexacoordinated boron species can be stabilized in a heterocyclic system by complexation with appropriate ligands. Compounds of this type (structures 2 and 3, Figure 3) were first reported by Lee and Martin in 1984 (1984JA5745). H, and NMR spectra of products 2 and 3 are consistent with the symmetrical structures incorporating pentacoordinated and hexacoordinated central boron atom however, X-ray data were not available in this publication. 

Figure 3 Pentacoordinated and hexacoordinated hypervaient boron species reported by Lee and Martin in 1984 (1984JA5745).

Muetterties and Schunn (1966) suggest that certain boron hydride structures, e.g. 26 and 22 or 23 (C, symmetry, when X=Y), might be used as models for electrophilic substitution. In general, if 21 or 22 were the preferred C3 transition states or intermediates, the reaction would go with retention. Note that these are the analogs of the two-electron three-center species A3 discussed earlier by HMO theory. If, however, X, Y, and R were permuted, there would be several stereochemical possibilities, two of which are indicated in Table 7. To decide what kind of geometry is assumed in SE2 transition states, which are pentacoordinated and electron- and orbital-deficient, calculations on model species are needed to establish preferred geometries. 

More recendy, Akiba and coworkers have synthesized pentacoordinated hypervalent boron heterocyclic species 4—6 by employing a sterically rigid anthracene skeleton (Scheme 1) (2000ACE4055,2005JA4354). Compounds 6—8 were prepared by lithiation of bromoanthracene 4 followed by the addition of E-chlorocatecholateborane derivatives 5. Products 6—8 were... 

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